Moving substrates of indefinite length (viz., moving webs) can be coated in controlled environments when ordinary ambient air conditions might disrupt the coating process or pose a safety hazard. Typical controlled environments include clean rooms and the use of inert, low oxygen or saturated atmospheres. Clean rooms and special atmospheres require costly auxiliary equipment and large volumes of filtered air or specialty gases. For example, a typical clean room operation may require many thousands of liters per minute of filtered air.
Conventional practices for the removal and recovery of components during drying of coated webs generally utilize drying units or ovens. Collection hoods or ports are utilized in both closed and open drying systems to collect the solvent vapors emitted from the web or coating. Conventional open vapor collection systems generally utilize air handling systems that are incapable of selectively drawing primarily the desired gas phase components without drawing significant flow from the ambient atmosphere. Closed vapor collection systems typically introduce an inert gas circulation system to assist in purging the enclosed volume. In either system, the introduction of ambient air or inert gas dilutes the concentration of the gas phase components. Thus the subsequent separation of vapors from the diluted vapor stream can be difficult and inefficient.
Additionally, the thermodynamics associated with the conventional vapor collection systems often permit undesirable condensation of the vapor at or near the web or coating. The condensate can then fall onto the web or coating and adversely affect either the appearance or functional aspects of the finished product. In industrial settings, the ambient conditions surrounding the process and processing equipment may include extraneous matter. In large volume drying units, the extraneous matter may be drawn into the collection system by the large volumetric flows of conventional drying systems.